Due to the terror, confusion and panic engendered by fire, it was recognized early on to provide the public with means for alerting it of the dangers posed by fire and smoke. Alarms communicating with electromechanical noise generating devices such as sirens and horns were first utilized. Since it is believed that calm directions during an emergency may tend to alleviate some of the panic, more sophisticated devices employed detectors in conjunction with record players and tape playback mechanisms to sound the alarm and issue voice commands. However, these record playing and tape playback mechanisms are cumbersome and are prone to failure.
Similarly, public areas usually include illuminated signs pointing the way to an emergency egress.
A major drawback with these devices is that oftentimes a power interruption accompanies a fire, plunging an area into darkness and perhaps disabling the alarm means as well. Accordingly, the exit signs and warning devices are equipped with their own self-contained backup power supply (batteries). Means are provided to maintain the batteries in a float (fully charged) condition. Upon the initiation of an appropriate signal (fire, smoke, gas leak, power failure, etc.), the device may switch over to consume its internal power supply in order to drive the lamps and the voice signals even when utility supplied current is lost.
Representative designs include U.S. Pat. Nos. 4,275,274; 4,107,464; 3,582,949; 2,479,528; 3,310,793 and 3,906,491 and U.K. Pat. No. 1,420,603.